Most of the definitions here are simply defining "disappointment". Anticipointment is trickier than that: it's anticipating something (usually eagerly) that you expect to be disappointed by on some level as well. It's not simply anticipation followed by disappointment; it's kinda both at the same time.
The classic example is the computer-user who eagerly anticipates a price-drop on new components, but knows that the price drops because of even newer technology - which they can't afford. The user is trapped in a continuing state of anticipointment.